Gray-scale operation on nodes
The luminance weights are intended to be summed. That is, for any given colourspace, multiply each light by its luminance position, and sum the result with the other two. You will end up with equal values across all three channels.

For REC.709 lights, which are the lights that define the sRGB specification, the formula would be:

Y = 0.2126*R + 0.7152*G + 0.0722*B